In bottom trawl fishing, the bottom trawl net is towed along the ocean bottom behind a towing vessel that is positioned at the surface of the ocean at a finite towing speed. Such towing speed varies, say from 1 to 7 knots per hour in operations, with about 3 knots per hour being preferred. The bottom trawl net forms an envelope with a closed back and sides and an open mouth for catching the fish. A floating system is attached to the upper leading edge of the net to keep the top of the trawl net a predetermined elevation above the ocean bottom at a specific towing speed. The lower leading edge of the net is dragged along the ocean floor via a sweepline. Due to the presence of rocks and other impediments, there is high likihood of that the trawl net will intersect and make damaging contact with the impediments during operations. However, experience has indicated there are a multiplicity of approaches to dynamically solving the damage problem to the trawl net. In some cases, the towing vessel increases speed to avoid a particularly rocky patch. Other times, the net is outfitted with conmbinations of rubber discs, washers, sleeves etc., to allow the same to operate despite the impediments. In static solving the damage problem, it is usual to repair the trawl net aboard the towing vessel before deploying the latter at sea.
In protecting the trawl net against damage, usually commercial fishing vessels are equipped with two types of sweepline gear: (i) generally, for sandy, gentle seabeds, roller gear is attached to a sweepline, such roller gear comprising a series of rollers such as described in U.S. Pat. No. 4,813,170 centrally disposed about the sweepline chain; and (ii) for more rugged, terrain, commonly a rock gear comprising rubber discs attached to the sweepline chain, the rubber discs being formed by a stamping operation in which the discs are formed from the treads of abandoned vehicle tires such as shown in U.S. Pat. No. 5,068,994. In the former, the roller gear is permitted to rotate relative to the sweepline, while in the latter, the rubber discs do not rotate but are used to absorb energy as contact with impediments occur. In addition, note in the latter situation, that a series of rubber straps can also be added in position along the U-shaped sweepline having termini in light contact with the ocean bottom since the straps are hinged and tiltable into a vertical position at a selected tow speed by the selection of a weight upstream to the straps that overcomes the drag force provided the trawl net by the tow lines. Such weight(s) are attached to a chain that extends through the centers of the straps. However, most commercial fishermen have large capital outlays in nets and sweeplines. Hence, modifications of such systems using a weighted system such as shown in U.S. Pat. No. 5,068,994 that requires re-calibrating the entire trawl and adding new sweepline elements, have been found to be costly and difficult to implement.
Other attempts to protect the trawl net against damage are shown in British Patent No. 349,8871 and Danish Patent No. 43,998 both assigned to V. D. Limited, London, England. In accordance with the patentee, weighted panels of netting are positioned to depend from the trawl net and contact the ocean bottom. However, since such panels do not include an auxiliary floatation system, such panels have been found to be difficult to implement. Moreover, since the weighted ends of the panels are adjacent to the ocean bottom, they also inflict damage on the ocean bottom and accept damage therefrom.